Common hippos are recognisable by their barrel-shaped torsos, wide-opening mouths revealing large canine tusks, nearly hairless bodies, columnar legs and large size; adults average 1,500 kg (3,310 lb) and 1,300 kg (2,870 lb) for males and females respectively, making them the largest species of land mammal after the three species of elephant and the white and Indian rhinoceros. Despite its stocky shape and short legs, it is capable of running 30 km/h (19 mph) over short distances.

The common hippopotamus inhabits rivers, lakes and mangrove swamps, where territorial bulls preside over a stretch of river and groups of five to thirty females and young. During the day, they remain cool by staying in the water or mud; reproduction and childbirth both occur in water. They emerge at dusk to graze on grasses. While hippopotamuses rest near each other in the water, grazing is a solitary activity and hippos are not territorial on land. The hippopotamus is among the most dangerous animals in the world as it is highly aggressive and unpredictable. They are threatened by habitat loss and poaching for their meat and ivory canine teeth.

Etymology

The Latin word "hippopotamus" is derived from the ancient Greekἱπποπόταμος, hippopotamos, from ἵππος, hippos, "horse", and ποταμός, potamos, "river", meaning "horse of the river".[3][4][5] In English, the plural is "hippopotamuses", but "hippopotami" is also used;[6] "hippos" can be used as a short plural. Hippopotamuses are gregarious, living in groups of up to thirty animals. A group is called a pod, herd, dale, or bloat.

The suggested subspecies were never widely used or validated by field biologists; the described morphological differences were small enough that they could have resulted from simple variation in nonrepresentative samples.[7]:2 Genetic analyses have tested the existence of three of these putative subspecies. A study examining mitochondrial DNA from skin biopsies taken from 13 sampling locations, considered genetic diversity and structure among hippo populations across the continent. The authors found low, but significant, genetic differentiation among H. a. amphibius, H. a. capensis, and H. a. kiboko. Neither H. a. tschadensis nor H. a. constrictus has been tested.[8][9]

The most recent theory of the origins of Hippopotamidae suggests that hippos and whales shared a common semiaquatic ancestor that branched off from other artiodactyls around 60 million years ago.[12][14] This hypothesized ancestral group likely split into two branches around 54 million years ago.[11]

One branch would evolve into cetaceans, possibly beginning about 52 million years ago, with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.[15] The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of which in the late Eocene would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants.[14]

A rough evolutionary lineage can be traced from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene species Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene.[16]Merycopotamus, Libycosaurus and all hippopotamids can be considered to form a clade, with Libycosaurus being more closely related to hippos. Their common ancestor would have lived in the Miocene, about 20 million years ago. Hippopotamids are therefore deeply nested within the family Anthracotheriidae.

The Hippopotamidae are believed to have evolved in Africa; the oldest known hippopotamid is the genus Kenyapotamus, which lived in Africa from 16 to 8 million years ago. While hippopotamid species spread across Asia and Europe, no hippopotamuses have ever been discovered in the Americas, although various anthracothere genera emigrated into North America during the early Oligocene. From 7.5 to 1.8 million years ago, an ancestor to the modern hippopotamus, Archaeopotamus, lived in Africa and the Middle East.[17]

While the fossil record of hippos is still poorly understood, the two modern genera, Hippopotamus and Choeropsis (sometimes Hexaprotodon), may have diverged as far back as 8 million years ago. Taxonomists disagree whether or not the modern pygmy hippopotamus is a member of Hexaprotodon – an apparently paraphyletic genus, also embracing many extinct Asian hippopotamuses, that is more closely related to Hippopotamus – or of Choeropsis, an older and basal genus.[16][17]

Choeropsis madagascariensis skeleton with a modern hippopotamus skull.

Extinct species

Three species of Malagasy hippopotamus became extinct during the Holocene on Madagascar, one of them within the past 1,000 years. The Malagasy hippos were smaller than the modern hippopotamus, likely through the process of insular dwarfism.[18] Fossil evidence indicates many Malagasy hippos were hunted by humans, a likely factor in their eventual extinction.[18] Isolated members of Malagasy hippopotamus may have survived in remote pockets; in 1976, villagers described a living animal called the kilopilopitsofy, which may have been a Malagasy hippopotamus.[19]

Description

Hippo's skull, showing the large canines and incisors used for fighting.

Hippopotami are among the largest living land mammals, being only smaller than elephants and some rhinoceroses. Mean adult weight is around 1,500 kg (3,310 lb) and 1,300 kg (2,870 lb) for males and females respectively,[22][23] very large males can reach 2,000 kg (4,410 lb) and exceptional males weighing 2,660 kg (5,860 lb)[22] and 3,200 kg (7,050 lb)[24] have been reported. Male hippos appear to continue growing throughout their lives while females reach maximum weight at around age 25.[25]

Mostly submerged hippo with exposed eyes, ears, and nostrils

Hippopotami have barrel-shaped bodies with short legs and long muzzles.[26] Their skeletal structures are graviportal,[7]:8 adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river.[27] Hippopotamuses have small legs (relative to other megafauna) because the water in which they live reduces the weight burden.[28] Though they are bulky animals, hippopotamuses can gallop at 30 km/h (19 mph) on land but normally trot. They are incapable of jumping but do climb up steep banks.[26] Despite being semiaquatic and having webbed feet, an adult hippo is not a particularly good swimmer nor can it float. It is rarely found in deep water; when it is, the animal moves by porpoise-like leaps from the bottom.[7]:3 The eyes, ears, and nostrils of hippos are placed high on the roof of their skulls. This allows these organs to remain above the surface while the rest of the body submerges.[29]:259 The testes of the males descend only partially and a scrotum is not present. In addition, the penis retracts into the body when not erect. The genitals of the female are unusual in that the vagina is ridged and two large diverticula protrude from the vulval vestibule. The function of these is unknown.[7]:28–29

Characteristic "yawn" of a hippo.

The hippo's jaw is powered by a large masseter and a well-developed digastric; the latter loops up behind the former to the hyoid.[29]:259 The jaw hinge is located far back enough to allow the animal to open its mouth at almost 180°.[7]:17 A moderate folding of the orbicularis oris muscle allows the hippo to achieve such a gape without tearing any tissue.[30] The bite force of an adult female has been measured as 8,100 newtons (1,800 lbf).[31] Hippopotamus teeth sharpen themselves as they grind together. The lower canines and lower incisors are enlarged, especially in males, and grow continuously. The incisors can reach 40 cm (1 ft 4 in), while the canines reach up to 50 cm (1 ft 8 in).[26] The canines and incisors are used for combat and play no role in feeding. Hippos rely on their broad horny lips to grasp and pull grasses which are then ground by the molars.[29]:259, 263 The hippo is considered to be a pseudoruminant; it has a complex three-chambered stomach but does not "chew cud".[7]:22

Unlike most other semiaquatic animals, the hippopotamus has very little hair.[29]:260 The skin is 6 cm (2 in) thick,[26] providing it great protection against conspecifics and predators. By contrast, its subcutaneous fat layer is thin.[7]:3 The animals' upper parts are purplish-gray to blue-black, while the under parts and areas around the eyes and ears can be brownish-pink.[29]:260 Their skin secretes a natural sunscreen substance which is red-coloured. The secretion is sometimes referred to as "blood sweat", but is neither blood nor sweat. This secretion is initially colourless and turns red-orange within minutes, eventually becoming brown. Two distinct pigments have been identified in the secretions, one red (hipposudoric acid) and one orange (norhipposudoric acid). The two pigments are highly acidic compounds. They inhibit the growth of disease-causing bacteria, and their light absorption peaks in the ultraviolet range, creating a sunscreen effect. All hippos, even those with different diets, secrete the pigments, so it does not appear that food is the source of the pigments. Instead, the animals may synthesize the pigments from precursors such as the amino acidtyrosine.[32] Nevertheless, this natural sunscreen cannot prevent the animal's skin from cracking if it stays out of water too long.[33]

Genetic evidence suggests that common hippos in Africa experienced a marked population expansion during or after the Pleistocene, attributed to an increase in water bodies at the end of the era. These findings have important conservation implications as hippo populations across the continent are currently threatened by loss of access to fresh water.[8] Hippos are also subject to unregulated hunting and poaching. In May 2006, the hippopotamus was identified as a vulnerable species on the IUCN Red List drawn up by the International Union for Conservation of Nature (IUCN), with an estimated population of between 125,000 and 150,000 hippos, a decline of between 7% and 20% since the IUCN's 1996 study. Zambia (40,000) and Tanzania (20,000–30,000) possess the largest populations.[1]

The hippo population declined most dramatically in the Democratic Republic of the Congo.[42] By 2005, the population in Virunga National Park had dropped to 800 or 900 from around 29,000 in the mid-1970s.[43] The decline is attributed to the disruptions caused by the Second Congo War.[43] The poachers are believed to be Mai-Mai rebels, poorly paid Congolese soldiers, and local militia groups.[43][44] Reasons for poaching include the belief that hippos are harmful to society, as well as financial gain.[45] However, as of 2016, the Virunga hippo population appears to have increased, possibly due to greater enforcement and cooperation between fishermen and park authorities.[46] The sale of hippo meat is illegal, but black-market sales are difficult for Virunga National Park officers to track.[44][45] Hippo meat is considered a delicacy in some areas of central Africa and the teeth have become a valued substitute for elephant ivory.[47]

Invasive potential

In the late 1980s, Colombian drug lord Pablo Escobar kept four hippos in a private menagerie at his residence in Hacienda Nápoles, 100 kilometres (62 mi) east of Medellín, Colombia, after buying them in New Orleans. They were deemed too difficult to seize and move after Escobar's death, and hence left on the untended estate. By 2007, the animals had multiplied to 16 and had taken to roaming the area for food in the nearby Magdalena River.[48][49]

In 2009, two adults and one calf escaped the herd and, after attacking humans and killing cattle, one of the adults (called "Pepe") was killed by hunters under authorization of the local authorities.[49][50] As of early 2014, 40 hippos have been reported to exist in Puerto Triunfo, Antioquia from the original four belonging to Escobar.[51] The National Geographic Channel produced a documentary about them titled Cocaine Hippos.[52] A report published in a Yale student magazine noted that local environmentalists are campaigning to protect the animals, although there is no clear plan for what will happen to them.[53]

Behaviour and life history

Hippopotamus out of the water after sunrise

Hippopotamus in the water

Different from all other large land mammals, hippos are of semiaquatic habits, spending the day in lakes and rivers.[7]:3 They can be found in both savannah and forest areas.[1] Proper habitat requires enough water to submerge in and grass nearby.[26] Larger densities of the animals inhabit quiet waters with mostly firm, smooth sloping beaches. Males may be found in very small numbers in rapid waters in rocky gorges.[29]:264 Hippo mostly live in freshwater habitats, however populations in West Africa mostly inhabit estuarine waters and may even be found at sea.[1] With the exception of eating, most of hippopotamuses' lives – from fighting with other hippos, mating, to parturition – occurs in the water. Hippos leave the water at dusk and travel inland, sometimes up to 10 km (6 mi),[26] to graze on short grasses, their main source of food. They spend four to five hours grazing and can consume 68 kg (150 lb) of grass each night.[54]

Hippopotamus feeding on grass

Like almost any herbivore, they consume other plants if presented with them, but their diet in nature consists almost entirely of grass, with only minimal consumption of aquatic plants.[55] Hippos are born with sterile intestines, and require bacteria obtained from their mothers' feces to digest vegetation.[56] Hippos have (albeit rarely) been filmed eating carrion, usually close to the water. There are other reports of meat-eating, and even cannibalism and predation.[57] The stomach anatomy of a hippo is not suited to carnivory, and meat-eating is likely caused by aberrant behaviour or nutritional stress.[7]:84

Hippo defecation creates allochthonous deposits of organic matter along the river beds. These deposits have an unclear ecological function.[55] A 2015 study suggests that hippo dung provides nutrients from terrestrial material for fish and aquatic invertebrates.[58] Because of their size and their habit of taking the same paths to feed, hippos can have a significant impact on the land across which they walk, both by keeping the land clear of vegetation and depressing the ground. Over prolonged periods, hippos can divert the paths of swamps and channels.[59]

Adult hippos move at speeds up to 8 km/h (5 mph) in water; typically resurfacing to breathe every three to five minutes. The young have to breathe every two to three minutes.[7]:4 The process of surfacing and breathing is spontaneous. A hippo sleeping underwater rises and breathes without waking up. A hippo closes its nostrils when it submerges into the water.[60] As with fish and turtles on a coral reef, hippos occasionally visit cleaning stations and signal, by opening their mouths wide, their readiness for being cleaned of parasites by certain species of fishes. This is an example of mutualism in which the hippo benefits from the cleaning, while the fish receive food.[61]

Hippopotamus coexist with a variety of formidable predators. Nile crocodiles, lions and spotted hyenas are known to prey on young hippos.[29]:273[7]:118 However, due to their aggression and size, adult hippopotamus are not usually preyed upon by other animals. Cases where large lion prides have successfully preyed on adult hippopotamus have been reported; however, this predation is generally rare.[62] Lions occasionally prey on adults at Gorongosa National Park and calves are commonly taken at Virunga.[63]Crocodiles are frequent targets of hippo aggression, probably because they often inhabit the same riparian habitats; crocodiles may be either aggressively displaced or killed by hippopotamuses.[64]

Social spacing

Hippopotamus pod

Studying the interaction of male and female hippopotami has long been complicated because hippos are not sexually dimorphic; thus females and young males are almost indistinguishable in the field.[65] Although hippos lie close to each other, they do not seem to form social bonds except between mothers and daughters, and they are not social animals. The reason they huddle close together is unknown.[7]:49 Hippopotamuses are territorial only in water, where a bull presides over a small stretch of river, on average 250 m (270 yd) in length, and containing 10 females. The largest pods can contain over 100 hippos.[7]:50 Younger bachelors are allowed in a bull's stretch, as long as they behave submissively toward the bull. The territories of hippos exist to establish mating rights. Within the pods, the hippos tend to segregate by gender. Bachelors lounge near other bachelors, females with other females, and the bull on his own. When hippos emerge from the water to graze, they do so individually.[7]:4

Males fighting

Hippos mark their territory by defecation. While depositing the faeces, hippos spin their tails to distribute their excrement over a greater area.[66] "Yawning" serves as a threat display.[26] When fighting, male hippos use their incisors to block each other's attacks and their large canines to inflict injuries.[29]:260 When hippos become over-populated or a habitat is reduced, bulls sometimes attempt infanticide, but this behaviour is not common under normal conditions.[67] Incidents of hippo cannibalism have been documented, but this is believed to be the behaviour of distressed or sick hippos.[7]:82–83

Hippopotami appear to communicate vocally, through grunts and bellows, and they may practice echolocation, but the purpose of these vocalizations is currently unknown. Hippos have the unique ability to hold their heads partially above the water and send out a cry that travels through both water and air; individuals respond above and under water.[68] Hippos will also express threat and alarm with exhalations.[26]

Reproduction

Female hippos reach sexual maturity at five to six years and have a gestation period of eight months.[69] A study of endocrine systems revealed that female hippopotamuses may begin puberty as early as three or four years.[70] Males reach maturity at around 7.5 yr. A study of hippopotami reproductive behaviour in Uganda showed that peak conceptions occurred during the end of the wet season in the summer, and peak births occurred toward the beginning of the wet season in late winter. This is because of the female's oestrous cycle; as with most large mammals, male hippopotamus spermatozoa is active year-round. Studies of hippos in Zambia and South Africa also showed evidence of births occurring at the start of the wet season.[7]:60–61 After becoming pregnant, a female hippopotamus will typically not begin ovulation again for 17 months.[70]

Preserved hippo fetus

Mating occurs in the water, with the female submerged for most of the encounter,[7]:63 her head emerging periodically to draw breath. Female hippos isolate themselves to give birth and return within 10–14 days.[26] Baby hippos are born underwater at a weight between 25 and 50 kg (55 and 110 lb) and an average length of around 127 cm (4.17 ft), and must swim to the surface to take their first breaths. A mother typically gives birth to only one calf, although twins also occur. The young often rest on their mothers' backs when the water is too deep for them, and they swim under water to suckle. They suckle on land when the mother leaves the water.[7]:64

Mother hippos are very protective of their young and may keep others at a distance. However, calves are occasionally left in nurseries which are guarded by one or a few adults. Calves in nurseries engage in playfights.[26]Weaning starts between six and eight months after birth, and most calves are fully weaned after a year.[7]:64 Like many other large mammals, hippos are described as K-strategists, in this case typically producing just one large, well-developed infant every couple of years (rather than many small, poorly developed young several times per year as is common among small mammals such as rodents).[70][67]

Hippos and humans

The earliest evidence of human interaction with hippos comes from butchery cut marks on hippo bones at Bouri Formation dated around 160,000 years ago.[71] Later rock paintings and engravings showing hippos being hunted have been found in the mountains of the central Sahara dated 4,000–5,000 years ago near Djanet in the Tassili n'Ajjer Mountains.[7]:1 The ancient Egyptians recognised the hippo as a ferocious denizen of the Nile and representations on the tombs of nobles show that the animals were hunted.[72]

The hippopotamus was also known to the Greeks and Romans. The Greek historian Herodotus described the hippopotamus in The Histories (written circa 440 BC) and the Roman naturalist Pliny the Elder wrote about the hippopotamus in his encyclopedia Naturalis Historia (written circa 77 AD).[40][73] The Yoruba people called the hippopotamus erinmi which means "elephant of the water".[74]Zulu warriors preferred to be as brave as a hippopotamus, since even lions were not considered to match its courage.[75] They would chant their chief: "Een-gonyama Gonyama! Invooboo! Yah-bo! Yah-bo! Invooboo!" which translates as "He is a lion. Yes, he is better than a lion – he is a hippopotamus."[76]

Attacks on humans

The hippopotamus is considered to be very aggressive and has frequently been reported as charging and attacking boats.[82] Small boats can be capsized by hippos and passengers can be injured or killed by the animals or drown. In one case in Niger, a boat was capsized by a hippo and 13 people were killed.[83] As hippopotamuses will often engage in raiding nearby crops if the opportunity arises, humans may also come in conflict with them on these occasions, with potential for fatalities on both sides.[84]

Hippos in zoos

Hippopotamuses have long been popular zoo animals. The first zoo hippo in modern history was Obaysch, who arrived at the London Zoo on May 25, 1850, where he attracted up to 10,000 visitors a day and inspired a popular song, the "Hippopotamus Polka".[85] Hippos generally breed well in captivity; birth rates are lower than in the wild, but this is attributed to zoos wanting to limit births, since hippos are relatively expensive to maintain.[7]:129[85][86]

Like many zoo animals, hippos were traditionally displayed in concrete exhibits. In the case of hippos, they usually had a pool of water and patch of grass. In the 1980s, zoo exhibits increasingly reflected native habitats. The Toledo Zoo Hippoquarium, features a 360,000 gallon pool.[87] In 1987, the Toledo Zoo saw the first underwater birth by a captive hippo.[88] The exhibit was so popular, the hippos became the logo of the Toledo Zoo.[89]

Cultural depictions

Ijaw hippopotamus masks

A red hippopotamus represented the Ancient Egyptian godSet; the thigh is the "phallic leg of Set" symbolic of virility. Set's consort Tawaret was also seen as part hippo[90] and was a goddess of protection in pregnancy and childbirth, because ancient Egyptians recognised the protective nature of a female hippopotamus toward her young.[91] The Ijaw people of the Niger Delta wore masks of aquatic animals like the hippo when practicing their water spirit cults[92] and hippo ivory was used in the divination rituals of the Yoruba.[93] The Behemoth from the Book of Job, 40:15–24 is thought to be based on a hippo.[94]

Hippopotamuses have been the subjects of various African folktales. According to a San story; when the Creator assigned each animal its place in nature, the hippos wanted to live in the water, but were refused out of fear that they might eat all the fish. After begging and pleading, the hippos were finally allowed to live in the water on the conditions that they would eat grass instead of fish and would fling their dung so that it can be inspected for fish bones. In a Ndebele tale, the hippo originally had long, beautiful hair, but was set on fire by a jealous hare and had to jump into a nearby pool. The hippo lost most of his hair and was too embarrassed to leave the water.[95]

The Hippopotamus Polka

Ever since Obaysch inspired the "Hippopotamus Polka", hippos have been popular animals in Western culture for their rotund appearance that many consider comical.[85] Stories of hippos such as Huberta, which became a celebrity in South Africa in the 1930s for trekking across the country;[96] or the tale of Owen and Mzee, a hippo and tortoise which developed an intimate bond; have amused people who have bought hippo books, merchandise, and many stuffed hippo toys.[97][98] Hippos were mentioned in the novelty Christmas song "I Want a Hippopotamus for Christmas" that became a hit for child star Gayla Peevey in 1953. They also feature in the songs "The Hippopotamus" and "Hippo Encore" by Flanders and Swann, with the famous refrain "Mud, Mud, Glorious Mud". They even inspired a popular board game, Hungry Hungry Hippos.[99][100]

^Snyder, Kristen Denninger (7 December 2015). "The Common Hippopotamus in the Wild and in Captivity: Conservation for Less Charismatic Species". Journal of International Wildlife Law & Policy. 18 (4): 337–354. doi:10.1080/13880292.2015.1096162.

^Melissa Greene (December 1987). "No rms, jungle vu: a new group of "landscape-immersion" zoo designers are trying to break down visitors' sense of security by reminding them that wild animals really are wild". The Atlantic Monthly.